Field of the Invention
The present invention is directed to automatic self-cleaning strainer assemblies which include a backwash cleaning system to remove debris from the screen and, more particularly, to automatic self-cleaning strainer assemblies which utilize a reduced amount of fluid during the backwash cleaning cycle.
Description of Related Art
Strainers and filters are employed to separate solids from fluids. The solids are collected on screening media or screens and, thereafter, the solids are removed from the screening media by backwashing. During backwashing, a portion of the fluid being strained is caused to flow tangentially across and in a reverse direction through the screen into a backwash port to clean the media, as described, for example, in U.S. Pat. Nos. 4,818,402, 5,152,891, and 5,595,655, the entirety of which are hereby incorporated by reference. As used herein, the term backwash port refers to a suction chamber, collection duct, or backwash port.
Typically, these types of strainers are installed on the discharge side of pumps which have operating pressures usually in excess of 10 psig with the backwash ports discharging to atmospheric pressure. These high differential pressures may be effectively used to backwash self-cleaning strainers. High differential pressures result in high fluid velocities across the screen in the backwashing mode of operation. The high fluid velocities remove debris collected on the screen. However, in applications which require the screen on the suction side of the system, due to space limitations and/or for pump protection, the differential pressure available may only be 5 psig or less. This normally is an insufficient pressure differential to effectively clean the screen. In some low pressure applications, an external source of backwash cleaning fluid can be applied to clean the screen and then removed via a backwash collection arrangement.
Although these backwashing methods are effective in removing the debris from the screen, whether the backwashing cycle uses the fluid being strained or clean, high pressure fluid, or a combination of both, these methods typically require a large amount of the cleaning fluid, usually 3-7% of the total fluid flows through the unit. This represents an excessive use of fluid which ultimately exits the device and is lost through the backwash cleaning arrangement.